Friday, February 17, 2012

Changing flood risk: When 1/500 is actually 1/240 – 1/25

A few years ago, I wrote a blog entry "When 1-in-1000 is no longer 1-in-1000", wondering a little bit about the language and implications surrounding increased periodicity and intensity of storms and flooding. At the time, I wrote:

If we are going to enter into a period of greater rainfall intensity -- and, consequently, more flash-flooding -- then the number of expected large floods should also increase. If we continue to term floods as 1000-year-floods (or 300-year-floods, or any other X-year-floods), then shouldn't they change to meet the new understandings of flooding? I mean, if a 1000-year-flood for a particular stretch of river is currently listed as a discharge of 10,000 cfs, that means that every year, there is a 1/1,000 chance that a discharge of 10,000 cfs will be reached. However, let's assume that climate change will change the hydrological impacts for that region, increasing the intensity of flooding such that a 10,000 cfs discharge has a 1/700 chance each year. That will mean that the same 10,000 cfs discharge will now be a 700-year-flood.

[R]esearchers from MIT and Princeton University have found that with climate change, [Category 3] storms could make landfall [in the eastern US] far more frequently, causing powerful, devastating storm surges every three to 20 years. The group simulated tens of thousands of storms under different climate conditions, finding that today’s “500-year floods” could, with climate change, occur once every 25 to 240 years. The researchers published their results in the current issue of Nature Climate Change.

Okay first things first. The stated range is quite wide: "25-year floods" to "240-year floods", an almost-10-fold difference. This is much like the difference between having $1 dollars in your wallet versus having $10 dollars there.

However, even the high-end range (240-year floods) is roughly half that of the estimates of current expectations (i.e., a 500-year flood). To extend the previous simile, this is much like expecting to have $50 dollars in your wallet, but finding that you only have $24 (or - to use the low-end figure - $2.50). In other words, the models predict that the frequency of such storms will be anywhere from 2x more likely to 10x more likely than presently expected.

If the findings of this study are borne out in the real world, the implications will be staggering. It will affect city and project planning, it will affect insurance rates, it will affect agriculture, it will even affect property boundaries as increased flooding will mean that rivers and coastlines will shift to accommodate the "new normal".

Some insights about what all these numbers mean in a real-world setting are explained in the news report:

Today, a “100-year storm” means a surge flood of about two meters [~6.5 ft], on average, in New York. Roughly every 500 years, the region experiences towering, three-meter-high [9.8-foot-high] surge floods. Both scenarios, Lin notes, would easily top Manhattan’s seawalls, which stand 1.5 meters [~4.9 ft] high.

But with added greenhouse gas emissions, the models found that a two-meter surge flood would instead occur once every three to 20 years; a three-meter flood would occur every 25 to 240 years.

“The highest [surge flood] was 3.2 meters [10.5 ft], and this happened in 1821,” Lin says. “That’s the highest water level observed in New York City’s history, which is like a present 500-year event.”

In other words, if NYC is going to get 10-foot-high flood surges twice as often (or even ten times as often), all that concern, panic, economic cost, etc. will occur more often. Furthermore, it will be even more likely that there will actually be a major flood that over-tops the seawalls and floods (perhaps even destroying) parts of Manhattan.

And that's something that any insurance company will have to worry about.